The rotary-type escapement for a watch or the like usually has a rotary balance carrying an eccentric pallet pin that is periodically and regularly oscillated forward and backward about a balance axis. As the balance oscillates its pallet pin engages on each forward oscillation between the teeth of an adjacent escape wheel rotatable about a respective wheel axis and displaces this wheel angularly one step. The teeth and pallet pin are shaped so that on reverse displacement of the pallet pin the escape wheel is moved backwardly to a minor extent insufficient to cancel the forward displacement of the escape wheel. A magnet or similar retaining device is provided to hold the escape wheel in position between forward oscillations of the balance.
The pallet pin is therefore oscillated back and forth through an arc. In only a portion of this arc, having an angular width equal to the so-called lift angle, does the pallet pin on forward oscillation engage against and displace the escape wheel. The overall oscillation angle is, however, considerably longer than the lift angle. Furthermore, the pallet pin only engages within the orbit of the outer ends of the teeth of the escape wheel during displacement through the so-called engagement angle. Within this engagement angle displacement of the escape wheel independently of the balance wheel is almost impossible, as the teeth would strike the pallet pin and their displacement would thereby be limited.
The principal problem with such escapements which are otherwise known for their simplicity and low manufacturing cost is that it is frequently possible for the escape wheel to advance more than one step for each forward oscillation of the pallet pin. This happens particularly in wrist watches which are subjected to occasional shocks that allow the escape wheel to rotate through several angular steps when the pallet pin is at the end of its travel and outside the engagement angle, that is lying outside the orbit of the teeth. This extra stepping most commonly occurs when the wrist watch is subjected to a shock at or shortly after the instant when during forward oscillation the pallet pin strikes the tooth of the escape wheel.